To be suitable for implantation within the human body, a prosthesis should be made of a material that is not physically or chemically degraded or altered by contact with body fluids, that is not toxic or allergenic to human tissue and that will not excite an inflammatory or foreign body reaction. Over the years, silicone elastomers have been found to be the materials best suited for implantation within the human body, because they come the closest to realizing the above-stated requirements. For this reason, silicone elastomers have been widely used for coating cardiac pacemakers, for making implanted catheters (Irving, Hall & Rickham, Tissue Reaction to Pure and Impregnated Silastic, Journal of Pediatric Surgery, Vol. 6, No. 6 at 724 (December 1971)), and for making mammary prostheses. The use of silicone elastomers in the manufacture of mammary prostheses has been especially extensive.
However, despite the relative inertness of silicone elastomers, they may still provoke an inflammatory or foreign body reaction in some patients to varying degrees. When a foreign substance enters human tissues, the immediate and natural reaction of the tissues surrounding the foreign substance is to render it harmless to the rest of the body. Small foreign bodies can be disposed of by phagocytosis, but large and inert foreign bodies are encapsulated in a sheath of fibrous tissue to isolate them from surrounding tissues. Encapsulation is a defensive mechanism that occurs through a process similar to the formation of scar tissue in the healing of a wound or surgical incision. A fibrous tissue capsule will form around and completely enclose an implanted prosthesis in an intimate fashion, conforming to its respective shapes and curvatures.
Capsule formation is not a problem for the patient unless the capsule begins to contract. With an implanted mammary prosthesis, the contracture of the capsule around the prosthesis causes it to be compressed tightly and feel very hard and rigid. Ultimately, the contracted capsule assumes a nearly spherical shape. Capsular contracture causes discomfort and embarrassment to the patients who experience it and is a serious condition both from medical and aesthetic viewpoints. One way to remedy capsular contracture is to surgically remove the contracted capsule and implant and then insert either the same or another implant, a procedure called surgical capsulotomy or capsulectomy. Alternatively, some doctors use closed capsulotomy, a method wherein force is applied to break the capsule in situ. Of course, capsular contracture can still recur.
The problem of capsular contracture is very complex and the reasons why it occurs are not yet fully understood. Nonetheless, several different approaches to avoiding capsular contraction have been investigated. One of the most popular approaches followed today involves the use of steroids. Steroids are known to possess anti-inflammatory and anti-fibrinogenic properties and have been observed to cause a decrease in the relative hardness of breasts implanted with mammary prostheses. However, the use of steroids can result in complications such as tissue atrophy and discoloration of the skin. Accordingly a great deal of controversy surrounds the use of steroids and their relative utility in preventing capsular contracture. Other drugs and techniques have also been suggested, but their utility has not yet been established.
Other approaches to the problem of capsular contracture have focused on the design of the implant. Examples of mammary prostheses designed to prevent or alleviate the effects of capsular contracture are disclosed in U.S. Pat. Nos. 3,189,921; 3,366,975; 3,559,214; 3,600,718; 3,934,274; 4,095,295; 4,298,997; 4,298,998; and 4,428,082. Of these, those receiving the greatest commercial acceptance are made of a flexible, thin-walled container or sac composed of a material impervious to the ingrowth of fibrous tissue, such as a silicone elastomer, to the external surface of which a thin layer of a porous or open-celled material has been adheringly applied (U.S. Pat. Nos. 3,366,975 or 3,559,214). The interior of the sac is filled with an inert material approximating the resiliency of normal mammary tissue, such as a saline solution or a silicone gel.
The porous or open-celled layer is normally composed of a polyether, polyester or polyurethane foam material. Thin layers of this type of material had been applied to the back sides of mammary prostheses so that fibrous tissue could grow into the material and thereby anchor a prosthesis securely to the chest wall (U.S. Pat. No. 3,293,663). However, case studies conducted on mammary prostheses almost completely covered with a thin foam layer indicated that the incidence of capsular contracture was reduced by the use of such prostheses (Pennisi, Polyurethane-Covered Silicone Gel Mammary prosthesis for Successful Breast Reconstruction, Aesthetic Plastic Surgery, Vol. 9 at 73 (1985); Ashley, Further Studies on the Natural-Y Breast Prosthesis, Plastic and Reconstructive Surgery, Vol. 45, No. 5 at 421 (May 1970)). Although the cause for the reduced incidence is not fully understood, it is believed that the growth of the fibrous tissue into the open-cell layer from many directions prevents the fibrous tissue from contracting in a concerted manner. In other words, the contractions occur in many directions and tend to neutralize each other (Pennisi, supra, at 73).
However, possible problems exist with the use of polyether, polyester or polyurethane foam materials in implants. These materials apparently degrade in the body over a period of time (Brown, Lowry and Smith, The Kinetics of Hydrolytic Aging of Polyester Urethane Elastomers, National Bureau of Standards (July 1979); Sherman and Lyons, The Biological Fate of Implanted Rigid Polyurethane Foam, Journal of Surgical Research, Vol. 9, No. 3 at 167 (March 1969)). Therefore, the effectiveness of these materials for preventing capsular contracture may disappear as they degrade When capsular contracture does occur and the doctor chooses to surgically remove the implant, it is difficult to ensure that all of the degraded material has been removed. These materials have also been suspected of creating problems with infection and of being carcinogenic.
To avoid the potential problems with existing foam materials and still take advantage of the reduced incidence of capsular contracture attendant with the use of prostheses having a porous or open-celled outer layer, ways have been sought to make a layer of silicone elastomer having an open-cell texture. In U.S. Pat. No. 3,852,832, a mammary prosthesis is disclosed having a fixation means attached to its back side with perforations passing therethrough and ribs projecting therefrom. This fixation means is preferably to be made of a silicone elastomer. Although no method for making such a fixation means is disclosed, it is believed that it would be separately molded. Therefore, the pattern of perforations and ribs would have to be such as to allow removal from a mold. The fixation means must then be attached to the prosthesis.
Ion-beam thruster technology has also been suggested as a way to microtexture breast prostheses (Picha and Siedlak, Ion-Beam Microtexturing of Biomaterials, MD & DI at 39 (April 1984)). However, this would interject an expensive processing step into the manufacture of breast prostheses. Also, it is not clear how the prostheses will be manipulated to achieve microtexturing over the entirety of their contoured surfaces or how effective the regular pattern of a microtextured surface will be at preventing capsular contracture. Other attempts to create an open-celled texture integral to the prosthesis shell through incorporating foaming or blowing agents in the silicone elastomer have not been successful because the surface properties of the silicone elastomers prevent the bubbles formed from connecting to one another or opening at the surface.
Accordingly, a need exists for a silicone elastomer medical implant having an external surface with an open-celled texture. Additionally, a need exists for an efficient and economic method for making such a medical implant.